Linear amplifiers are used in a variety of applications, including, e.g., audio, video, and power supplies. Linearity between an amplifier input signal and the corresponding output signal enables faithful reproduction of analog information presented at the amplifier input. A key metric associated with linearity and amplifier quality is slew rate, often expressed as volts per second (millivolts per microsecond, etc.). Slew rate is a measure of the maximum rate of change of the information signal at a particular point within the amplifier. A poor slew rate may result in a distorted output, as fast-changing portions of the information signal are delayed in time relative to other portions of the signal.
One application for which amplifier slew rate may be particularly important is that of regulating the output voltage of a switching DC-DC voltage converter. A switching DC down-converter (known in the art as a “buck” converter) alternately makes and breaks a circuit path between the converter DC supply input and an energy conversion inductor. During the ON state, the inductor stores energy in a magnetic field as current flows through the inductor. During the OFF state, the collapsing magnetic field generates current at the converter output. The inductor thus integrates the switched waveform to create an output voltage waveform proportional to the duty cycle of the active-state switched waveform. A filter capacitor is typically used to smooth the voltage waveform at the converter output.
To maintain a voltage set-point at the output of a DC-DC converter as load current demand changes, the output voltage level may be monitored and fed back to a circuit controlling the switching duty cycle. The slew rate of the DC-DC converter feedback circuit may be particularly important in applications requiring tight voltage regulation. For example, modern processors typically employ hundreds of millions of transistors. The on/off state of each transistor contributes to the overall instantaneous current load presented to the DC power supply. The instantaneous current load may be quite high and may change substantially within a few microseconds. And, low-voltage operation associated with today's microprocessor technology presents an additional challenge to DC power supply regulation requirements, given that small supply voltage variations may represent a significant percentage of a processor's operating voltage margin.
The slew rate of components associated with the output voltage controlling feedback loop in a DC-DC converter may limit the regulation precision of the converter. Signals applied to the differential inputs of the error amplifier typically include a reference voltage and a voltage divided sample of the converter output. The feedback error signal appears at the output of the error amplifier. Some error amplifiers use a “dynamic bias” current circuit and provide the error signal in the form of a variable current source. The magnitude of the output current source is proportional to the magnitude of the difference between the reference voltage and the converter output sample voltages appearing at the error amplifier differential inputs.